Patients sometimes experience stenosis in an anatomical structure. Stenosis occurs when an abnormal narrowing or stenotic lesion appears in the anatomical structure. Physicians generally evaluate the stenotic lesion before selecting a therapy to treat it. For example, in the case of blood vessels, if the stenotic lesion obstructs blood flow through the vessel to a large degree, physicians often elect to place a stent within the lesion site. On the other hand, if the stenotic lesion only minimally obstructs blood flow, physicians sometimes elect not to use a stent.
One technique for evaluating the degree to which a stenotic lesion obstructs flow through a blood vessel is called the Fractional Flow Reserve measurement (FFR). To calculate the FFR for a given stenotic lesion, two blood pressure readings are taken. One pressure reading is taken on the distal side of the lesion (e.g., downstream from the lesion) and the other pressure reading is taken on the proximal side of the lesion (e.g., upstream from the lesion, towards the aorta). The FFR is defined as the ratio of maximal blood flow in a stenotic lesion, taken distal to the lesion, to normal maximal flow, and is typically calculated based on a measured pressure gradient of the distal pressure to the proximal pressure. The FFR is therefore a unitless ratio of the distal and proximal pressures. The pressure gradient, or pressure drop, across a stenotic lesion is an indicator of the severity of the stenosis, and the FFR is a useful tool in assessing the pressure drop. The more restrictive the stenosis is, the greater the pressure drop, and the lower the resulting FFR.
The FFR measurement is considered a useful diagnostic tool. For example, clinical studies have shown that an FFR of less than about 0.8 or about 0.75 can be a useful criterion on which to base certain therapy decisions. A physician might decide, for example, to perform an interventional procedure (e.g., angioplasty or stent placement) when the FFR for a given stenotic lesion is below 0.8 or 0.75, and may decide to forego such treatment for lesions where the FFR is above 0.8 or 0.75. Thus, the FFR measurement provides a decision point for guiding treatment decisions.
Certain drawbacks are sometimes seen with the FFR method. First, the FFR method is designed merely to determine whether an interventional procedure such as a stent is needed or not needed. It does not provide any tools for enabling a physician to select a stent size that is ideal for the specific stenotic lesion at issue. Stents come in a variety of sizes, and physicians generally need to select an appropriate size and shape depending on the lesion characteristics. Physicians often need to use a separate procedure to determine what stent size to use. Commonly, physicians use an intravascular ultrasound method to determine a diameter of the vessel having the stenotic lesion. This method involves the advancement of a separate ultrasound catheter and the use of a separate ultrasound machine. This adds significant cost and time and more risk to the patient. Thus, it would also be desirable to provide a more simple system capable of both obtaining FFR measurements and selecting an appropriate stent size.
Another drawback seen with the FFR method is that the presence of a measuring device itself in the anatomical structure can affect the accuracy of the measurement. For example, as the measuring device crosses the stenotic lesion, the device itself introduces flow obstruction, in addition to that caused by the lesion itself. The measured distal pressure is sometimes lower than it would be without the additional flow obstruction, which may exaggerate the measured pressure gradient across the lesion. Thus, it would also be desirable to provide an improved system for obtaining more accurate FFR measurements.